Power over Ethernet (PoE) technology is directed to passing electrical power, along with data, on Ethernet cabling. PoE technology is typically regulated by the IEEE 802.3 standard. Power is supplied in common mode over two or more of the differential pairs of wires found in the Ethernet cabling and comes from a power supply within a PoE-enabled networking device, such as an Ethernet switch, or can be injected into a cable run with a midspan power supply.
The basic elements of a PoE system include power sourcing equipment (PSE), which provides power on the Ethernet cable, and a powered device (PD) powered by the power sourcing equipment that consumes energy from the power sourcing equipment over the Ethernet cabling. Examples of powered devices include wireless access points, Internet protocol (IP) telephones, and IP cameras.
The power sourcing equipment performs a detection procedure to detect whether the powered device is a valid powered device that may be provided with power. The power sourcing equipment determines if the powered device is valid based on receiving a valid detection signal from the powered device. For example, the detection signal has a maximum resistance of 26.3 kohms and a maximum capacitance of 120 nF.
In accordance with the IEEE 802.3-2012 standard, the power sourcing equipment uses a maintain power signature (MPS) operation to determine if a powered device continues to require power after the power was provided to the powered device. The maintain power signature requires the powered device to periodically draw at least 10 mA.
One approach for a maintain power signature operation is provided by an electronic device 20 that includes a rectifier bridge 30 comprising an input 32 configured to be coupled to PoE power sourcing equipment via Ethernet cabling 34, and an output 36, as illustrated in FIG. 1. A transistor 40 selectively connects and disconnects the output 36 with a load 50. A parasitic diode 48 is associated with the transistor 40. When the transistor 40 is connected to the supply voltage (VDD) leg, a charge pump 60 is used to increase the voltage being applied to a control terminal 41 of the transistor 40.
The load 50 may be a power converter, such as a DC/DC converter, that generates a power output required by the powered device. The powered device is connectable to the power converter. The load 50 includes a load capacitance 52 as a low frequency filter. The load capacitance 52 may have a value of 100 μF, for example, and is charged to the supply voltage (VDD).
The electronic device 20 includes a maintain power signature device 70 which is configured as a current source to be connected across the rectifier bridge 30. When the load 50 periodically draws less than 10 mA, then a control circuit 80 is configured to enable the maintain power signature device 70 by generating a control signal to a maintain power signature switch 72 coupled in series with the maintain power signature device 70. With the maintain power signature device 70 enabled, the current consumption of the maintain power signature device increases the total current so as to reach 10 mA. In this configuration, the effect of a voltage droop is current sharing from the Ethernet cabling 34 via the rectifier bridge (path 1) and from the load capacitance 52 (path 2). However, more current is provided by the load capacitance 52 instead of from the power sourcing equipment because of the rectifier bridge 30.
Consequently, there is a need to cancel the current from the load capacitance 52 and provide an adequate flow of current from the power sourcing equipment over the Ethernet cabling 34.
The current IEEE 802.3-at standard allows for a maintain power signature current consumption of 10 mA for a minimum of 75 msec and for a maximum cycle of 250 msec. A voltage droop in the supply voltage (VDD) from the power sourcing equipment allows the load capacitance 52 to discharge current for a time period that reduces duration of the pulse of current for the maintain power signature.